1. Field of the Invention
The invention relates to the application of a metal-comprising coating to a metal surface. More particularly, it relates to a fluxless solder coating and joining process.
2. Description of the Prior Art
Solder joining typically is a process in which at least two metal surfaces to be joined are contacted with solder at a temperature above the melting point of the solder. To obtain good wetting and hence good bonding of the surfaces with the solder, the surfaces and the solder must be clean and free of compounds that would prevent such wetting. Such compounds that prevent wetting include, but are not limited to, oxides, chlorides, sulfides, carbonates and various organic compounds. The major impediment to wetting in most solder joining processes is an oxide coating on the solder and metal surfaces. The oxide coating typically results from the exposure of the solder and metal surfaces to air. It is necessary that such oxides and other contaminants be reduced to their metal form, reacted to form other compounds not detrimental to wetting, or removed by known means such as dissolution or mechanical cleaning, to assure good wetting.
Compounds detrimental to wetting are commonly reacted or dissolved and washed away by the use of a fluxing agent. Fluxing agents, however, are typically corrosive, and necessitate the removal residues thereof after the soldering process. The cleaning processes used to remove the flux residues are expensive and difficult to control. In addition, the most commonly used cleaning agent, CFC 113, has been shown to destroy ozone in the stratsophere. This presents a serious environmental problem, and most industrial nations have agreed, under the so-called Montreal protocol, to eliminate the production of this cleaning agent. All of the proposed alternatives to CFC 113 are either more expensive, not as effective, not as safe to use, or require the purchase of new cleaning equipment.
For such reasons, it is often desirable to employ a fluxless solder coating and joining process. If no flux is used, then no cleaning is required. In the Nowotarski patent, U.S. Pat. No. 4,821,947, such a process for fluxless coating and joining with solder is disclosed. In this process, solderable metal surfaces are contacted with an inert molten metal bath. While a flux must be used in conventional soldering in air in order to dissolve oxides and contamination from the solder bath and the solderable surface, the need for a flux is eliminated in the Nowotarski process by inerting the solder bath and surface, with the inert bath being oxide free. Contamination on the solderable surface is pulled away by the surface tension of the inert solder bath. This desirable action is enhanced if there is a coating on the solderable surface that is a liquid at soldering temperatures. The contamination that forms on the surface of the coating is then more easily pulled away from the solderable surface. A very common and effective protective coating is electroplated solder.
While the Nowotarski process represents a desirable advance in the art, it is nevertheless difficult with this process to control the amount of solder left on the solderable surface or joint after the solderable surface has detached from the solder bath. For example, if an electronic circuit board with a conductor pad were lowered into a solder bath under an inert atmosphere, with the conductor pad having an electroplated solder coating that is contaminated with solder oxides and dirt, the electroplated solder coating would melt and solder surface tension would pull the oxides and dirt away. The solder in the solder bath would then effectively wet the conductor pad. Upon pulling the circuit board away from the solder bath, however, the solder surface tension at the conductor pad would cause solder to be pulled up and cling to the conductor pad. The pressure inside the solder column would drop as the pulling away of the circuit board from the solder bath is continued, and the solder column would neck in and eventually break at the so-called point of departure. The solder above the point of departure would stay with the conductor pad, and the solder below would fall back into the solder bath.
The exact point at which the solder breaks would depend on many factors, including the circuit board withdrawal speed, the slope of the conductor pad, the surface tension of the solder and the like. It is very difficult to control all of these parameters so as to determine the final solder volume and shape in the coating. Consequently, the amount of solder in this and in other such . applications will be found to vary considerably. This high degree of sensitivity of the coating process to various operating parameters is a reflection of the fact that the solder separation process is very non-equilibrium. The pressure inside the column of solder changes suddenly from negative to positive when the solder column breaks.
While significant advances have been made in the soldering art, it will be appreciated that further improvements are desired to enhance the feasibility and acceptance of soldering without flux for a variety of practical commercial applications. Thus, the ability to control the final solder volume and shape would be of major benefit in the art. This would enable a desirable reproducibility and quality control to be achieved in such commercial applications.
It is an object of the invention, therefore, to provide an improved solder coating and joining process, and an improved solder coating and joint.
It is another object of the invention to provide a solder coating process in which the final solder volume and shape can be controlled.
It is a further object of the invention to provide a fluxless or nearly fluxless process for solder coating and joining with a controlled solder volume and shape. With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.